The invention is directed to a device and to a method for fixing a toner image on a carrier material, whereby the toner image is charged with a solvent vapor. The invention is also directed to an apparatus for printing and/or copying wherein such a device is utilized.
In numerous electrophotographic printing or copying processes, a toner image is transferred onto a carrier material, for example paper, the toner image being initially not joined to the carrier material in smear-proof and abrasion-resistant fashion. That the toner image is firmly joined to the carrier material, i.e. fixed, is only achieved by a fixing process. A fixing process is usually employed wherein the toner is charged with heat and pressure. The toner is thereby melted with the assistance of heating fixing drums and pressed into the carrier material, so that the toner enters into a bonded union with the carrier material. When no specific pre-heating of the paper is undertaken, this heat-pressure fixing is limited to the transport velocity of the carrier material, for example to approximately 0.5 m/s through 0.7 m/s.
When the carrier material is simultaneously printed on both sides in the operating mode of duplex printing and both sides are to be simultaneously fixed, then fixing drums that are soft and yielding must be employed at both sides. Such fixing drums have only a short service life and, due to the slight economic feasibility, are only utilized in printers or copiers having a low printing volume. Due to the resilience of the fixing drums, further, the guidance of the carrier material is problematic, so that an endless carrier material web can only be conditionally employed given such a fixing method.
Contactless fixing methods have already been proposed that avoid the problems arising due to the contact between carrier material and parts of the fixing mechanism, for example the fixing drums.
EP-A-0 629 930 discloses an arrangement wherein toner is melted with infrared radiation and is fixed on the paper. Such an arrangement can also be employed in duplex printing, whereby toner images are simultaneously fixed on both sides of the carrier material. When switched on and off, the infrared radiators that are employed have a relatively great time constant, so that a start/stop mode cannot be realized with such an arrangement without spoilage or rejects.
DE-A-198 27 210 discloses an arrangement wherein infrared radiation is likewise employed for fixing. A start/stop mode without rejects can be realized by means of the design control of a blind that is inserted into and in turn withdrawn from the beam path of the infrared radiation. However, the general disadvantage of fixing with the assistance of infrared radiation remains, this being comprised such that the carrier material, generally paper, is relatively intensely dried during the fixing event, this leading to a shrinkage, to ripple and to an electrostatic charging given further-processing and post-processing of the carrier material. Such a modification of the carrier material can lead to considerable problems in the post-processing of the carrier material, for example when cutting, stacking, binding, enveloping, etc.
Another known contactless fixing method is photoflash fixing wherein the toner is fixed on the carrier material with high-energy light pulses. The wavelength of the radiation generally lies in the visible through ultraviolet range of the radiation spectrum. Photoflash fixing reacts sensitively to the color of the toner, i.e. the toner material absorbs the energy dependent on the existing light spectrum, which can lead to quality losses given employment of toners having different color, for example in multi-color printing.
Another contactless fixing method is what is referred to as cold fixing. In this cold fixing, the toner material is softened under the influence of a solvent. The softened toner thereby moistens the carrier material. Given employment of fiber material that contains fibers such as, for example, paper or textiles, the softened toner surrounds the fibers and, due to capillary forces, penetrates between the fibers and into them. After the evaporation of the solvent, the toner in turn congeals and solidifies. In this way, the toner is joined to the carrier material in a smear-proof and abrasion-resistant manner. The presence of the solvent in vapor form during the fixing process is more advantageous than the presence as an aerosol or liquid, since chemical solvent processes sequence on a molecular basis and a molecular distribution of the solvent is thus the most suitable. Given the employment of vapor, moreover, a condensation of the solvent vapor onto the toner particles occurs due to the different temperatures of carrier material and vapor, so that the vapor molecules deposit directly onto these toner particles. Moreover, the output of the evaporation enthalpy in the condensation supports the softening of the toner and increases the speed of the dissolving process.
A general advantage of fixing with the assistance of a solvent is the slight thermal stressing of the carrier material. Accordingly, carrier materials can be employed that withstand only a slight thermal or mechanical load such as, for example, labels or films. Moreover, the moisture content of the carrier material is not changed, so that a ripple, a bagging or a curling arising due to changes in moisture are avoided. Moreover, cold fixing is largely independent of the thickness of the carrier material, so that, for example, papers having different paper thicknesses can be employed without a great modification of the fixing process. In this way, a change in the type of paper can also occur with little expense.
U.S. Pat. No. 4,311,723 discloses an arrangement wherein a paper web is conducted through a fixing chamber with solvent vapor. The solvent vapor is situated in a container. Due to the force of gravity as well as cooling tubes in the upper region of the container, the concentration of the solvent vapor increases toward the floor of the container, so that a region with a high solvent concentration arises in the proximity of the container floor. The carrier material, which enters in the upper region of the container with the as yet unfixed toner images, is deflected downward at a first deflection device and is conducted in the region of the high solvent concentration in the proximity of the container floor. The carrier material with partially fixed toner images is deflected again thereat at a second deflection device and is ultimately conducted upward out of the container via a third deflection device. Of necessity, a touching of the carrier material occurs at the deflection devices, as a result whereof the toner situated thereon can smear or peel off or print locations are left behind. It is therefore not possible with this arrangement to fix carrier material charged with toner images on both sides. Moreover, the arrangement exhibits a relatively slow start/stop behavior since—for stopping the fixing—the deflection device must be moved out of the region of high solvent concentration upward into a region having a low solvent concentration with which a fixing no longer occurs, a certain time being required for this.
The employment of solvent can be problematical in view of the creation of ozone. One speaks of the ozone potential of a solvent in this context. In U.S. Pat. No. 4,311,723, an azeotropic mixture of tri-chlorofluorethane (C2Cl2F3, CFC1130 and acetone (C6H6O) is employed. The primary solvent is the acetone, whereas the CFC113 serves as a flame retardant. The use of CFC113 was outlawed in the earlier 1990s due to the high ozone potential. Partially halogenated hydrocarbons, what are referred to as HCFC, were then proposed as a replacement for the CFC113, for example HCFC123 and HCFC141b, since these have a significantly lower ozone potential. These partially halogenated hydrocarbons HCFC henceforth assumed the function of the flame retardant in mixtures of air and combustible solvents such as acetone, propyl alcohols, etc. In particular, the use of pure HCFC141b without addition of a solvent such as, for example, acetone proved advantageous given employment of polystyrol-based toners since HCFC141b has an adequate fixing action for these toners and is simple to recover as a single-phase material since no mixing or de-mixing problems occur.
Due to its ozone potential and the environmental pollution produced as a result thereof, however, HCFC141b will only be available for a limited time. New fixative mixtures on the basis of chlorine-free, fully halogenated hydrocarbons HFC were therefore proposed, for example in EP-A-0 784 238 (Solvay) and EP-A0 941 503 (Allied Signal). Given the polyester based toners that are usually employed now, however, these mixtures have proven problematic to employ in practice.
EP-A-0 613 572 discloses a method and a solvent for fixing a toner constructed on the basis of polystyrol. A partially halogenated fluorohydrocarbon having a temperature of ebullition below 35° C. is employed as sole a solvent.
DE-A-2 720 247 discloses a printing process wherein toner is transferred from an intermediate carrier, for example a photoconductor drum onto a recording medium at a transfer printing station. The toner on the photoconductor drum is charged with a solvent vapor such that it becomes sticky. The recording medium, for example the paper, is likewise exposed to the solvent vapor. The sticky toner adheres on the paper, which is likewise provided with solvent, as a result whereof the transfer printing event and the fixing of the toner occur in a single process.
DE-A-2 613 066 discloses a fixing process for fixing toner images on paper. Hot gas with a predetermined proportion of water steam is employed for the non-contacting fixing.
DE-A-2 613 066 also discloses a fixing device wherein the toner material of the toner image on the recording medium is charged with a hot gas, particularly air. The temperature of this gas is such that the toner melts and penetrates into the recording medium, for example paper.
The following documents are cited in the International Search Report: CH,A,457 144; U.S. Pat. No. 3,680,795; “Cut-Sheet Vapor Fuser”, IBM Technical Disclosure Bulletin, IBM Corp. New York 32 (3A), 1989, 258-259, XP000049471; DE,A,29 27 453; DE,A,36 36 324, with CH, A, 457144 being relevant.